TNguyen,

I recently purchased and installed a jump seat for my 1982 500sl (euro). I bought it from World Upholstery, but I think that they are all about the same. It installs easily, you just pull out the board that is there now, and the place the cushions in the well, they fit. The back cushion hooks over the back. No tools involved. However, couple of things you should know:
1. It comes with lap seatbelts. There is a hole already in place in the car to screw them in, but there should be 2 holes for each side, and there aren't. So, you either need to drill another hole, or attach both belts to the same one.
2. The seats are very small and narrow, and there is little to no room for legs. I don't know about your kids, mine are small but they ride in car seats/boosters. The seats are too narrow to hold a car seat, and a booster should only be used with an over the shoulder seatbelt, not a lap belt. I have tried to solve this by ordering the only additional belt that works with older cars that only have a lap belt in the back seat (ie older cars). This additional belt is called a 86-y universal harness, and can be ordered from e-z-on products in florida. Haven't installed them yet, so I can't tell you how they work.

Good luck,

Kevin